Spread of non-native grasses into grazed versus ungrazed desert grasslands

Livestock grazing may or may not be an exogenous disturbance facilitating the spread of exotic grasses, perhaps depending on the historical importance of native ungulates in a particular grassland. We compared canopy cover of native grasses and two African lovegrasses (Eragrostis spp.) over 22 years in ungrazed versus livestock grazed desert grasslands in southeastern Arizona. The exotics comprised <1% of the total grass canopy in either treatment in 1984, but by 2006 this number had risen to 24% in grasslands ungrazed since 1968, and to 65% on adjacent cattle ranches. The exotics increased from 79% to 99% canopy over the same time period in ungrazed areas where they had been planted. Results indicate that: (1) protection from grazing reduced the rate of exotic invasions into native grasslands; (2) areas deliberately planted with the exotics developed into near monocultures even under livestock exclusion, and (3) livestock grazing is an exogenous disturbance in southeastern Arizona, to which exotics are better adapted than most native grasses. It remains to be determined whether ungrazed native desert grasslands will resist further incursions by the African species.